1. Field of the Invention
The present invention relates to an improved process for the liquefaction of raw coal. More particularly, the invention relates to a process wherein subdivided coal is dissolved in a process derived solvent having a low heptane-insolubles content and is subsequently hydrocracked under specified process conditions.
2. Prior Art
Coal is our most abundant indigenous fossil fuel resource, and as a result of dwindling petroleum reserves, concerted research efforts are being directed toward recovery of liquid hydrocarbons from coal on a commercial scale. A promising approach in this field is the direct liquefaction of coal accompanied with minimum gas production.
This approach has principally evolved from the early work of F. Bergius, who discovered that transportation fuels could be produced by the high pressure hydrogenation of a paste of coal, solvent and catalyst.
Later discoveries revealed the advantage of using specific hydrogenation solvents at lower temperatures and pressures. With these solvents, such as partially saturated polycyclic aromatics, hydrogen transfer to the coal is facilitated and dissolution enhanced. However, the products from single-stage dissolvers are typically high in asphaltenes, have high average molecular weights and high viscosities. These qualities present considerable obstacles in removing the fine coal residue particles suspended in the product which usually range from 1 to 25 microns in diameter.
The complete nature of the coal residue or undissolved solids is not wholly understood, but the residue appears to be a composite of organic and inorganic species. The residue organic matter is similar to coke and the inorganic matter is similar to the well known coal-ash constituents. The removal of these particles is of course necessary to produce a clean-burning, low ash fuel.
As a result, numerous researchers have focused their efforts upon devising methods to facilitate residue removal by nonconventional techniques. One of the approaches advocated is the addition of a precipitant or anti-solvent to the residue laden product. Suitable precipitating agents include aliphatic or naphthenic hydrocarbons. These agents are miscible with the liquefaction solvent but do not dissolve the coal residue which is thereby precipitated. U.S. Pat. Nos. 3,852,182 and 4,075,080, incorporated herein by reference, are representative examples of the prior art teachings in this area.
The use of such anti-solvents or precipitating agents is expensive from both capital and operating standpoints, however, and the procedure still suffers from an additional disadvantage. The product liquids from single stage dissolvers are usually high in asphaltenes. Traditionally asphaltenes have been defined as hydrogen-deficient high molecular weight hydrocarbonaceous materials which are insoluble in straight-chain aliphatic hydrocarbons such as n-heptane. It is now recognized that the broader definitions of asphaltenes relate to a wide spectrum of hydrocarbonaceous material which may be further characterized. A heptane insoluble asphaltene may be further extracted by using benzene, chloroform and dimethyl formamide solvents in that order. The benzene soluble asphaltenes are characterized with a high proportion of molecules having a molecular weight in the range of from about 450 to about 650 and only mildly hydrogen-deficient. The chloroform soluble asphaltenes are characterized with a high proportion of molecules having a molecular weight in the range of from about 1000 to about 1200. The DMF soluble asphaltenes are characterized with a high proportion of molecules having a molecular weight in the range from about 1800 to about 2000 and are severely hydrogen deficient. In a typical coal liquefaction extract the benzene, chloroform and DMF soluble asphaltene fractions would be expected to be about 50, 35 and 15 volume percent, respectively, of the heptane insoluble asphaltene fraction.
As used in the specification and claims herein this spectrum of high molecular weight hydrocarbonaceous compounds will be generically referred to as heptane-insolubles to avoid confusion with the traditional definition of asphaltenes, which would exclude the benzene insoluble materials.
Although asphaltenes are soluble in the coal solvents employed, they tend to precipitate from solution upon the addition of short-chain anti-solvents. Their precipitation aids in the agglomerization of the insoluble ash but results in substantial product loss from the high-boiling fractions of the dissolved coal. A recognition of this problem and an attempt to solve it is aptly illustrated in U.S. Pat. No. 4,029,567, also incorporated herein by reference.
J. Gatsis and G. Tan, apparently recognizing the above problem, proceeded to attack it from a different angle in U.S. Pat. No. 4,081,360, incorporated herein by reference, by suppressing asphaltene formation during the coal liquefaction step. The patent teaches liquefying coal with a low asphaltene hydrogenated coal solvent and then adding a light aromatic solvent to aid in ash separation. Other teachings to the same effect, include U.S. Pat. Nos. 3,997,425, 4,081,358, 3,081,359, 4,082,643 and 4,082,644.
Direct two-stage coal liquefaction processing evolved by the addition of a catalytic stage to further hydrogenate and break down the higher molecular weight products produced in the dissolver. In retrospect, and with the clarity hindsight often provides, such a step does not seem unprecedented. However, the direct passage of a solids-laden stream through a catalytic reactor was theretofor considered impractical at best. The two-stage units solved most of the coal residue removal problems since the hydrocracked product was relatively light and of relatively low viscosity, thereby permitting the use of conventional solids removal techniques. The asphaltene content of the product effluent from the catalytic reactor was drastically reduced by the catalytically induced hydrogenation. Representative patents covering two-stage coal liquefaction processes include U.S. Pat. No. 4,018,663 issued to C. Karr, Jr. et al, U.S. Pat. No. 4,083,769 issued to R. Hildebrand et al and U.S. Pat. No. 4,111,788 issued to M. Chervenak et al.
U.S. Pat. No. 4,018,663 discloses a two-stage process in which a coal-oil slurry is passed through a first reactor containing a charge of porous, non-catalytic contact material in the presence of hydrogen at a pressure of 1,000 to 2,000 psig and a temperature of 400.degree. to 450.degree. C. The effluent from this reactor is then preferably filtered to remove the coal residue and passed to a catalytic reactor for desulfurization, denitrification and hydrogenation of the dissolved coal.
U.S. Pat. No. 4,083,769 discloses a process wherein a preheated coal-solvent slurry is passed with hydrogen through a first dissolver zone operated at a pressure in excess of 3,100 pounds per square inch and at a higher temperature than the preheater. The dissolver effluent is then hydrogenated in a catalytic zone also maintained at a pressure in excess of 3,100 pounds per square inch and at a temperature in the range of 700 to 825.degree. F. to produce liquid hydrocarbons and a recycle solvent.
U.S. Pat. No. 4,111,788 discloses a process wherein a coal-oil slurry is passed through a dissolver containing no catalyst and the effluent therefrom is subsequently treated in a catalytic ebullated bed at a temperature at least 25.degree. lower than the temperature of the dissolver. A portion of the product liquid is preferably recycled for use as solvent.